In manufacturing electrolytic copper, electrolytic refining is usually performed in a sulfuric acid based electrolyte, using blister copper refined to a copper grade of about 99%. Noble metals such as gold and silver contained as impurities in blister copper are precipitated as anode slime. The profit in the entire process is enhanced by increasing the recovery rate of the noble metals.
If the noble metals are incorporated in electrolytic copper without migration into the anode slime, a loss of noble metals as products is caused. A useful technique for decreasing the Ag grade in electrolytic copper is therefore required. At the present time, in Saganoseki Smelter and Refinery of Pan Pacific Co., Ltd., the silver grade (silver content) in electrolytic copper is about 10 ppm. It is estimated that the decrease in silver grade to 5 ppm increases the production of silver by 1 ton per year.
As the cause of incorporation of Ag into electrolytic copper, the mechanical “entanglement” of anode slime may be considered, and the reductive electrodeposition of Ag ions dissolved in an electrolyte at cathode may also be considered. Examples of the known methods for decreasing the Ag grade in electrolytic copper by electro-sliming Ag ions in an electrolyte include adding a small amount of chloride ions to an electrolyte, so that Ag is precipitated and collected as electrolytic slime in a silver chloride form. In the method, the chloride ion concentration in the electrolyte is set to higher than 30 mg/L and 60 mg/L or less, and the temperature of an electrolyte in the vicinity of cathode is adjusted to 55° C. or lower, so that the solubility of silver chloride is decreased to accelerate the sliming of silver ions (Patent Literature 1).
Although a method for suppressing the Ag grade in electrolytic copper is known, as described above, the elution mechanism of Ag is not sufficiently known. Patent Literature 2 proposes to maintain the dissolved oxygen in an electrolyte at 3.0 mg/L or less, in manufacturing high-purity electrolytic copper by reelectrolysis of electrolytic copper as an anode in a sulfuric acid electrolytic bath. However, the technique has different conditions from those for manufacturing electrolytic copper from blister copper as an anode.